Data communication apparatus

Abstract

A data communication apparatus wherein the stealthiness has been enhanced by significantly increasing the time required for a wiretapper to decrypt an encrypted text. The data communication apparatus is constituted by connecting a data transmitting apparatus and a data receiving apparatus via a transmission path. The data transmitting apparatus receives a first predetermined initial value (key information) and information data, generates a multi-valued signal the level of which varies substantially like a random number, and converts the multi-valued signal to a modulated signal of a predetermined modulation format for transmission. The data receiving apparatus demodulates the modulated signal to output the multi-valued signal, and then reproduces the information data from the multi-valued signal and a second predetermined initial value (key information) that is received.

1 - 98 . (canceled) 
     
     
         99 . A data transmitting apparatus that performs cipher communication, comprising:
 a multi-level code generating part that generates, from predetermined key information, a multi-level code sequence a signal level of which varies substantially like a random number;   a first modulating part that generates a first modulated signal of a predetermined modulation format based on information data;   a second modulating part that generates a second modulated signal of a predetermined modulation format based on the multi-level code sequence,   wherein by cascading the first modulating part and the second modulating part, a modulated signal a modulation condition of which varies substantially like a random number is generated in correspondence with a combination of signal levels of the information data and the multi-level code sequence, and
 signal points of multiple levels in the modulated signal are substantially uniformly arranged. 
   
     
     
         100 . A data communication apparatus in which a data transmitting apparatus and a data receiving apparatus perform cipher communication,
 wherein the data transmitting apparatus comprises:   a first multi-level code generating part that generates, from predetermined key information, a first multi-level code sequence a signal level of which varies substantially like a random number;   a first modulating part that generates a first modulated signal of a predetermined modulation format based on information data;   a second modulating part that generates a second modulated signal of a predetermined modulation format based on the multi-level code sequence,   by cascading the first modulating part and the second modulating part, a modulated signal a modulation condition of which varies substantially like a random number is generated in correspondence with a combination of signal levels of the information data and the multi-level code sequence,   the data receiving apparatus comprises:   a second multi-level code generating part that generates, from key information equal to the key information, a second multi-level code sequence a signal level of which varies substantially like a random number;   a demodulating part that demodulates the modulated signal, and reproduces the multi-level signal; and   a multi-level discriminating part that discriminates the multi-level signal based on the second multi-level code sequence according to a predetermined processing, and reproduces the information data, and
 signal points of the multi-level signal are substantially uniformly arranged.